1. Field of the Invention
The invention relates to a mobile device, and more particularly to a mobile device which is able to detect a distance between a user and the mobile device, to adjust radio power thereof, so as to decrease specific absorption rate (SAR).
2. Description of the Related Art
With development of wireless communications technology, mobile devices are popular. For the 3rd generation mobile telecommunications (3G) technology, a Wideband Code Division Multiple Access (W-CDMA) system can provide circuit switched (CS) services and packet switched (PS) services to respectively process the transmitting/receiving of audio and data, such that applications of the mobile devices can be more diversified. However, according to recent research related to mobile devices, electromagnetic waves radiating from mobile devices (e.g. mobile phone) in use may have some negative effects for users. In general, mobile devices having external antennas will radiate more electromagnetic waves than those having built-in antennas.
Therefore, many countries have regulations for an electromagnetic specific absorption rate (SAR) value of a mobile device for human bodies, wherein SAR is a measure of the rate at which radio frequency (RF) energy is absorbed by a human body when exposed to an RF electromagnetic field. It is defined as the power absorbed per mass of tissue and has units of Watts per kilogram. For example, the maximum SAR value is 1.6 W/kg in average for Federal Communications Commission (FCC) regulations in the U.S.A.
Nowadays, mobile devices are used so popularly and decreasing the SAR for human bodies due to the mobile devices has become an important issue when designing the mobile devices. Usually, by increasing the size of circuit board or decreasing the radio power, the SAR of a mobile device is decreased. However, increasing the size of circuit board is contrary to the trend for miniaturization. Furthermore, decreasing the radio power decays signals transmitted by the antenna. Similarly, using a microwave absorbing material to change the electric field intensity distribution of a mobile device, also decays the signal transmitted by the antenna. Additionally, cost of the microwave absorbing material is high, thus increasing the cost of mobile devices.